Industrial processes result in sulfur oxides evolving from materials of the product being manufactured, for example in the manufacturing of bricks in a brick kiln, sulfur oxides evolve from the clay. The present invention is for use with any process in which sulfur oxides result, in order that they might be prevented from being released to the atmosphere. For example, Flue Gas Desulfurization (FGD) scrubbing systems are used in many industrial applications in which fossil fuels, containing sulfur, are combusted in order to provide heat required for various processes. Electrical power plants, for example, which burn coal, fuel oil, gas and the like, use FGD for removing sulfur oxides from the combustion gases, so as to prevent contamination of the atmosphere.
In addition to preventing pollutants from entering the atmosphere, chemical reaction products, resulting from chemical reactions taking place in a gas scrubbing system can be a valuable source of material for the manufacture of commercial by-products, for example gypsum.
A typical scrubber tower is shown in FIG. 2 having a vertically elongated scrubbing chamber 1 through which the gas to be desulfurized flows. A gas duct 2 delivers the gas at a lower section of the scrubbing tower, and an exhaust gas duct 3 removes the treated gas from an upper section of the scrubbing tower. Intermediate the ducts a series of sprays 4, or the like, distribute scrubbing slurry for reacting with the sulfur oxide pollutants contained in the gas. Following such reactions, a slurry reaction product drops, by gravity, to a bottom portion of the tower which includes a collection section 5 for holding the slurry reaction product. In order to provide and replenish chemicals for the reactions, such as MgSO3, Ca(OH)2, Mg(OH)2 and the like for reacting with the sulfur oxides of the gas, an alkaline slurry of such chemicals which can be prepared in mixing tank 6 is introduced into collection section 5. The material in collection section 5 is thus a scrubbing slurry containing liquid and solid phases of reactants and reaction products.
In order to maintain a desired level of scrubbing slurry in the collection section, a bleed stream 7 removes the scrubbing slurry from the collection section 5 for possible use in making commercial by-products.
A typical commercial by-product is gypsum. In a process for making gypsum the scrubbing slurry from bleed stream 7 is delivered to an oxidizer in which air or oxygen is combined with the scrubbing slurry in order to oxidize solid and liquid phase sulfites to sulfates. To promote dissolution of solid phase calcium sulfite in the oxidizer, the pH within the oxidizer should be less than 6.0 and preferably about 5.0. In the process of producing gypsum, once solid phase calcium sulfite is dissolved, oxidation of sulfites to sulfates occurs quickly and gypsum can be readily precipitated into a crystalline form.
However, in addition to the calcium sulfite, other alkaline solid phase constituents enter the oxidizer as well, that can raise the pH unless they are neutralized with an acid. These constituents are typically calcium carbonate from the alkaline slurry, unslaked calcium oxide or magnesium oxide, and magnesium hydroxide found with gypsum, that may be recycled from downstream processes. In order to maintain a favorable oxidizer pH, these alkaline constituents must be neutralized with an acid, preferably sulfuric acid.
Typically, there exists in the scrubbing slurry some soluble bisulfite (HSO3−1) that when oxidized will release an associated hydrogen ion. Released hydrogen ions then can neutralize the alkaline constituents that enter the oxidizer. Typically though, there is not enough acid associated with the bisulfites entering the oxidizer to neutralize all the alkaline constituents that enter the oxidizer, and additional acid is required to be added to the oxidizer to keep the pH within a range that allows oxidation to proceed quickly, as discussed above.
U.S. Pat. No. 6,695,018 describes a FGD system having a scoop disposed within the scrubbing tower and above an internal reaction tank, for collecting an effluent slurry of partially reacted scrubbing reagent and scrubbing reaction product, before they drain into the internal reaction tank. The partially reacted scrubbing reagent and scrubbing reaction product can then be withdrawn for use in manufacturing commercial by-products before the addition of fresh reagent raises the pH of the slurry of the internal reaction tank.
Although the collected effluent slurry of U.S. Pat. No. 6,695,018 may have a pH less than that of the slurry of the internal reaction tank, use of the withdrawn partially reacted scrubbing reactant and scrubbing reaction product may still require the addition of an acid in an oxidizer to adjust the pH to a level more favorable for the dissolution of solid phase calcium sulfite in the oxidizer.